Acidizing method

ABSTRACT

Subterranean formations are treated with acid, by a method comprising the steps of: (1) forming an acidizing composition by mixing an aqueous acid component with at least one water-soluble organosilicon compound; and (2) injecting the composition into the formation. Suitable organosilicon compounds include water-soluble organosilanes and organosilanes which hydrolyze to form water-soluble silanols. By using the organosilicon compound, movement of formation fine particles is inhibited and swelling of clays by aqueous fluids is reduced, thus tending to maintain formation permeability.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the introduction of fluids into subterraneanformations, more particularly to acid treatment of the formations.

2. Description of the Art

Acid treatment, or acidizing, is a very wellknown method for increasingor restoring the permeability of porous subterranean formations, for thepurpose of facilitating the flow of fluids, such as crude oil, naturalgas, geothermal fluids, and the like, through the formation. In atypical treatment, an acid or mixture of acids is introduced into aformation, through a well which penetrates the formation, usingsufficient pressure to obtain a desired distance of penetration into theformation. During the treatment, passageways for the flow of fluids areenlarged and a certain amount of new passageways may be formed.Acidizing effects depend primarily upon the chemical nature of theformation and the acid used, and typically are shown by an increase influid production (or fluid injection) rate through the well, afteracidizing has been conducted.

Materials which are attacked by the introduced acid can be originalcomponents of the formation, and also can be subsequently deposited bywell drilling, production, and injection operations after the well hasbeen completed. Very common carbonate-containing formations and cloggingdeposits are frequently treated with hydrochloric acid. Hydrofluoricacid, a mixture of hydrochloric and hydrofluoric acids, or a mixture ofhydrochloric acid and fluoride salts, all of which can be generallydescribed by the term "mud acid," is commonly utilized for removingsiliceous materials.

Strong, aqueous mineral acids, however, react almost instantaneouslywith the first reactive materials encountered as the acids areintroduced into a formation. Frequently, these reactive materials arefinely divided particles from the formation itself. One undesired resultis the consumption of large amounts of acid very near the wellbore,through which acid is injected, and a limited acid penetration into theformation. Adequate penetration depths are achieved only by the use ofvery large quantities of acid, which itself causes added corrosionproblems for well tubing and other subsurface equipment, due to theirincreased exposure to acidic materials.

These disadvantages have been addressed, at least in part, by addingvarious materials to the acid which make the acid less reactiveinitially, but which maintain reactivity over a prolonged period as theacid moves into a formation. Such added materials generally form: (1)emulsions with aqueous acid solutions; or (2) polymeric thickened orgelled acid compositions. Factors such as formation heat are used todecompose the emulsion or polymer, releasing reactive acid in a more orless gradual manner.

U.S. Pat. No. 4,479,543 to L. J. Kalfayan and D. R. Watkins describes adeeper penetrating acidizing method, in which the acid solutioninjection is preceded by injecting a slug of an organosilane or an esterof an organosilane. Preferably, the organosilane or ester is injected asa solution in a hydrocarbon carrier liquid, to prevent water contactbefore the material enters the formation to be treated, sincewater-reacted silane material was thought to penetrate a formation onlyto a limited extent. One possible mechanism proposed to explain theenhanced acidizing effects observed from use of the method is thecoating of formation fines with silane material, which polymerizes andprotects the fines against acid attack.

The previously discussed methods for acidizing procedure improvementhave the common disadvantage of increased complexity, as compared tonormal, simple acid injection techniques. Additional time, energy,equipment, and materials are required to form emulsions which are stableenough for acidizing, and polymer-thickened acids are more difficult toinject into a formation. Multiple injection procedures also suffersomewhat from increased labor and equipment utilization costs.

SUMMARY OF THE INVENTION

The invention is a method for acidizing subterranean formations,comprising forming an acidizing composition by mixing an aqueous acidwith at least one water-soluble organosilicon compound, and injectingthe composition into the formation.

Aqueous acids which are useful in the invention include mineral acids,organic acids, and mixtures thereof. Fluoride-containing "mud acids" areuseful in the practice of the invention.

Organosilicon compounds which are useful include those generallyregarded as having considerable water solubility, such as the aminosilanes, as well as organosilane materials which hydrolyze in an aqueousenvironment to form water-soluble silanols.

The organosilicon compound acts to retard the rate of reaction betweenthe acid and components of the subterranean formation, and also reducesthe mobility of fine particulate matter in the formation after acidizinghas been completed.

DETAILED DESCRIPTION OF THE INVENTION

The invention pertains to methods for acidizing subterranean formations,employing a composition comprising an aqueous acid component and atleast one water-soluble organosilicon compound.

Acid components which are suitable for the practice of the inventioninclude aqueous solutions of mineral acids, organic acids, and mixturesthereof. As is known in the art, fluorine-containing acids can be usedto treat formations which contain siliceous materials, whilenon-fluorine acids are more typically used to treat formations which arepredominantly non-siliceous. Where it is desired to treat formationssuch as a carbonate-containing sandstone, where carbonate minerals wouldrapidly consume fluorine-containing acids, before significant reactionoccurs with silica or silicate materials, two separate acid injectionsinto the formation are frequently used: first, a non-fluorine acid isinjected to react with carbonates; and subsequently, afluorine-containing acid is injected to react with silica and/orsilicates. The method of this invention can be used in any of theseprocedures, including various modified procedures which are known in theart.

The mineral acids include, without limitation, hydrochloric acid, nitricacid, hydroiodic acid, hydrobromic acid, sulfuric acid, sulfamic acid,phosphoric acid, mixtures of any of the foregoing acids with one or morewater-soluble fluoride salts, hydrofluoric acid, fluoboric acid,hexafluorophosphoric acid, difluorophosphoric acid, fluorosulfonic acid,and mixtures thereof.

The organic acids include, without limitation, formic acid, acetic acid,halogenated derivatives of acetic acid, citric acid, propionic acid,tartaric acid, and mixtures thereof.

Many additives are commonly used in acidizing solutions, such ascorrosion inhibitors, surface active agents, viscosity-modifying agents,and the like. These materials are also useful in the practice of theinvention and can be present in the acidizing composition.

Suitable water-soluble organosilicon compounds for the inventioninclude, without limitation, amino silanes such as3-aminopropyltriethoxy silane and N-2-aminoethyl-3-aminopropyltrimethoxysilane, and vinyl silane compounds such as vinyl tris-(2-methoxyethoxy)silane. However, as discussed by M. R. Rosen, "From Treating Solution toFiller Surface and Beyond. The Life History of a Silane Coupling Agent,"Journal of Coatings Technology, Vol. 50, No. 644, pages 70-82 (1978),many organosilane compounds are water-soluble for prolonged periods oftime after they hydrolyze to form silanols, and acids can serve to aidthe hydrolysis. For purposes of the present invention, then, compoundswhich form water-soluble silanols by hydrolysis will be considered asequivalent to the originally water-soluble organosilicon compounds.

Among the organosilanes suitable for use in this invention are thosehaving the formula: ##STR1## wherein X is a halogen, R₁ is an organicradical having from 1 to 50 carbon atoms, and R₂ and R₃ are the same ordifferent halogens or organic radicals having from 1 to 50 carbon atoms.Preferably, X is a halogen selected from the group consisting ofchlorine, bromine and iodine with chlorine being preferred, R₁ is analkyl, alkenyl, or aryl group having from 1 to 18 carbon atoms and R₂and R₃ are the same or different halogens, or alkyl, alkenyl, or arylgroup having from 1 to 18 carbon atoms.

Suitable specific organosilanes include methyldiethylchlorosilane,dimethyldichlorosilane, methyltrichlorosilane, dimethyldibromosilane,diethyldiiodosilane, dipropyldichlorosilane, dipropyldibromosilane,butyltrichlorosilane, phenyltribromosilane, diphenyldichlorosilane,tolyltribromosilane, methylphenyldichlorosilane, and the like.

Among the esters of the organosilanes suitable for use in this inventionare those having the formula: ##STR2## wherein R₄, R₅, and R₆ areindependently selected from hydrogen and organic radicals having from 1to 50 carbon atoms, provided not all of R₄, R₅, and R₆ are hydrogen, andR₇ is an organic radical having from 1 to 50 carbon atoms. Preferably,R₄, R₅, and R₆ are independently selected from hydrogen, amine, alkyl,alkenyl, aryl, and carbhydryloxy groups having from 1 to 18 carbonatoms, with at least one of the R₄, R₅, and R₆ groups not beinghydrogen, and R₇ is selected from amine, alkyl, alkenyl, and aryl groupshaving from 1 to 18 carbon atoms. When R₄, R₅, and/or R₆ arecarbhydryloxy groups, alkoxy groups are preferred.

Suitable specific esters of organosilanes include methyltriethoxysilane,dimethyldiethoxysilane, methyltrimethoxysilane, divinyldimethoxysilane,divinyldi-2-methoxyethoxy silane, di(3-glycidoxypropyl) dimethoxysilane,vinyltriethoxysilane, vinyltris-2-methoxyethoxysilane,3-glycidoxypropyltrimethoxysilane, 3-methacryloxypropyltrimethoxysilane,2-(3,4-epoxycyclohexyl) ethyltrimethoxysilane,N-2-aminoethyl-3-propylmethyldimethoxysilane,N-2-aminoethyl-3-propyltrimethoxysilane,N-2-aminoethyl-3-aminopropyltrimethoxysilane,3-aminopropyltriethoxysilane, and the like.

The acidizing composition is prepared by mixing the components,preferably in an acid-resistant container. Components may be added tothe container in any desired order.

In the composition, the acid component usually comprises about 0.5 toabout 50 percent by weight, more preferably about 5 to about 50 percentby weight, and the organosilicon component usually comprises about 0.1to about 10 percent by weight, although, of course, the solubility limitof the component should not be exceeded.

For practicing the method of the invention, it is sometimes preferred topre-flush the formation by injecting salt solutions, particularly whenthe formation connate water is quite hard and the acidizing compositioncontains components which form precipitates with ions in the water, toreduce precipitation of insoluble materials when the acidizingcomposition contacts the formation. Useful salt solutions include,without limitation, aqueous sodium chloride or ammonium chloridesolutions. Hydrocarbon liquids are also useful for a pre-flush of theformation. These liquids, which can include aromatic solvents, are usedto dissolve organic materials, such as wax and heavy oil, from mineralor scale surfaces to permit reaction with the acid. The liquids alsoserve to separate the acid from crude oil, helping prevent the formationof sludges or emulsions caused by the acid-oil oil interaction. Thesolvent can also contain a glycol ether compound, usually inconcentrations of 5 to 10 percent, to help remove emulsion blocksdownhole or to leave mineral and scale surfaces water-wet to aid theirreaction with acid. Ethylene glycol monobutyl ether is an example ofcompounds which can be used. The volume of preflush is typically about 1to about 500 gallons per vertical foot of formation to be treated.

Acidizing is conducted by injecting the composition through a well intothe formation, using pressures sufficient to penetrate the formation.Typical volumes of acidizing composition used are about 1 to about 500gallons per vertical foot of formation to be treated. Penetration can beimproved by following this injection with injecting into the formationan afterflush fluid, typically comprising an aqueous solution of a salt,such as ammonium chloride, or a liquid which is immiscible with theacidizing composition, such as a hydrocarbon liquid (crude oil, dieselfuel, kerosene, and the like). The acidizing composition itself andafterflushes often contain a glycol ether compound, such as ethyleneglycol monobutyl ether. The glycol ether tends to prevent emulsionblocks and to retard adsorption of other acidizing additives onto theformation face. However, it does not prevent the reaction of thewater-soluble organosilanes with the formation. When used in anafterflushing fluid, such as a hydrocarbon liquid, the glycol ether canaid in the removal of acidizing additives, such as corrosion inhibitors,which may have adsorbed on the formation and could restrict flow offluids through the formation. The afterflush assists in displacing theacidizing composition into the formation, and is typically about 1 toabout 500 gallons per vertical foot of formation to be treated.

After the acidizing composition has had sufficient time to react withthe formation, the composition is removed from the formation through thewell. Following the removal operation, the well can be used for itsnormal desired purpose, e.g., production from, or injection into, theformation.

While the reaction of the organosilicon compound with materials in theformation is not completely understood, and while the invention is notto be held to any theory of operation, it is believed that theorganosilicon compound condenses on and reacts with active sites onsiliceous surfaces, with which it comes in contact, to form a polymer.It is believed that a silane monomer first hydrolyzes and forms areactive intermediate and either the acid or alcohol depending on thetype of monomer: ##STR3##

The reactive intermediates, "silanols," then condense to begin formationof the polymer. ##STR4##

The growth of the polymer can proceed as hydrolysis and condensationcontinue.

The silanol can also react with active sites on the rock to covalentlybind the polymer to it: ##STR5## The polymer becomes covalently bondedto any siliceous surface, including clays and the quartz grains whichdefine the pore structure in sandstones or poorly consolidated orunconsolidated formations containing siliceous materials. The polymeracts as a "glue" to bind formation fines in place, thus reducing theirmovement when a fluid flows through the formation and decreasing theirreactivity toward acids. The polymer also coats any water-swellableclays and thereby reduces their subsequent swelling by water-containingfluids.

For purposes of the present invention, "formation fines" are defined asparticles small enough to pass through openings of the smallest sievecommonly available (400 U.S. Mesh, or 37 micron openings). Thecomposition of the fines can be widely varied as there are manydifferent materials present in subterranean formations. Broadly, finesmay be classified as being quartz, or other minerals such as: feldspars;muscovite; calcite; dolomite; barite; water-swellable clays, includingmontmorillonite, beidellite, nontronite, saponite, hectorite andsauconite (with montmorillonite being the clay material most commonlyencountered); non-water-swellable clays, including kaolinite and illite;and amorphous materials. Fines are present to some extent in mostsandstones, shales, limestones, dolomites and the like. Problemsassociated with the presence of fines are often most pronounced insandstone-containing formations.

When acidizing solutions contain the organosilicon compounds, a retardedrate of reaction between acids and formation components is obtained. Inaddition, the apparent coating effect of organosilicon compound reactionproducts persists after the acidizing solution is no longer present,resulting in stabilization of the fine particulate matter againstmovement in the formation and a greater retention of the increasedpermeability obtained by acidizing.

The invention is further illustrated by the following examples which areillustrative of various aspects of the invention and are not intended aslimiting the scope of the invention as defined by the appended claims.In the examples, all percentage composition values are expressed on aweight basis.

EXAMPLE 1

An acidizing composition is prepared by mixing together a 15 percentaqueous solution of hydrochloric acid, with sufficient3-aminopropyltriethoxysilane to comprise 1 percent by weight of thesolution. Similarly, an aqueous solution containing 12 percenthydrochloric acid and 3 percent hydrofluoric acid is mixed withsufficient 3-aminopropyltriethoxysilane to comprise 1 percent of thesolution.

In both cases, a clear solution is produced, which remains stable andclear, and does not exhibit appreciable changes in viscosity, uponstorage in a closed container for over 30 days. Further, it is observedthat the capacity of the acid for dissolving finely divided,acid-soluble particles is not significantly diminished by addition ofthe silane.

EXAMPLE 2

An acidizing solution is prepared by mixing an aqueous solutioncontaining 10 percent hydrochloric acid and 0.5 percent hydrofluoricacid with sufficient 3-aminopropyltriethoxysilane to comprise 0.5percent of the solution. The solution is heated to about 200° F. andplaced in contact with a piece of glass for about 40 minutes. No visibleetching of the glass is evident, but a very small amount of insolubleprecipitate is noted in the solution.

A similar acidizing solution is prepared, but omitting the silane. Thissolution is heated to about 200° F. and placed in contact with glass forabout 40 minutes, after which considerable etching of the glass isapparent, along with a large amount of insoluble precipitate in thesolution.

The example demonstrates the retarded acid reaction rate which isobtained by adding organosilicon compounds to acidizing compositions, inaccordance with the invention.

EXAMPLE 3

A cylindrical core sample of a sandstone material, which contains fineparticles of hydrochloric acid-soluble siderite, is used for a test ofthe present invention. The sample is mounted in a high-pressure coreholder, after being subjected to a vacuum of 50 torr for about 120minutes, then being saturated with a 2 percent aqueous solution ofsodium chloride. An overburden pressure of about 1,000 p.s.i.g. isapplied to the sample and liquids are passed through the sample ineither a predesignated "production" or "injection" direction, using apulseless pump.

Results are summarized in Table I. Fluid "A" is an aqueous solutioncontaining 10 percent hydrochloric acid, 1.5 percent citric acid, and0.2 percent of a corrosion inhibitor. Fluid "B" is similar to Fluid A,additionally containing 1 percent of 3-aminopropyltriethoxysilane. Theseresults show that a formation which suffers permeability damage fromcontact with water (step 2) can be stimulated by acidizing, but is againdamaged by contact with water (step 6). However, treatment with asilane-containing acid not only improves permeability, but preventssignificant damage from subsequent water contact (step 9).

                                      TABLE I                                     __________________________________________________________________________             Flow  Flow Rate                                                                           Volume                                                                             Temp.                                                                             Permeability                                                                         % of Original                            Step                                                                             Fluid Direction                                                                           (cc/min)                                                                            (cc) (°F.)                                                                      (millidarcies)                                                                       Permeability                             __________________________________________________________________________    1  2% NaCl                                                                             Production                                                                          5.0   100  75  126    100                                      2  H.sub.2 O                                                                           Production                                                                          5.0    20  75   7.86  6.3                                      3  2% NaCl                                                                             Production                                                                          5.0   400  75   7.01  5.6                                      4  A     Injection                                                                           1.0    50  139 --     --                                       5  2% NaCl                                                                             Production                                                                          5.1   100  75  20.8   16.6                                     6  H.sub.2 O                                                                           Production                                                                          5.3   100  75   8.51  6.8                                      7  B     Injection                                                                           1.0    50  75  --     --                                       8  15% NaCl                                                                            Production                                                                          5.3   100  75  18.6   14.8                                     9  H.sub.2 O                                                                           Production                                                                          5.3   300  75  15.2   12.1                                     __________________________________________________________________________

EXAMPLE 4

Another sample of the sandstone of Example 3 is subjected to testing,using the same apparatus and general procedures.

Results are summarized in Table II. Fluid "C" is similar to Fluid B ofthe preceding example, additionally containing 10 percent of ethyleneglycol monobutyl ether.

The results further demonstrate the ability of the invention to protectformations against permeability losses from contacting water.

EXAMPLE 5

A sample of the sandstone of Example 3 is subjected to testing, as inthat example.

Results are summarized in Table III. Fluid "D" is a solution of 10percent by weight ethylene glycol monobutyl ether in kerosene; Fluid Bis as described in Example 3.

These results demonstrate the utility of the invention for protectingundamaged formations against the effects of water contact. It should benoted that even the formation permeability to kerosene flow is somewhatimproved by the injection treatment.

EXAMPLE 6

An additional sample of the sandstone of Example 3 is subjected totesting, using the apparatus, Fluid B, and general procedure of thatexample.

Results are summarized in Table IV. The results show improvement information permeability to both aqueous and organic liquids, followingtreatment according to the invention.

                                      TABLE II                                    __________________________________________________________________________             Flow  Flow Rate                                                                           Volume                                                                             Temp.                                                                             Permeability                                                                         % of Original                            Step                                                                             Fluid Direction                                                                           (cc/min)                                                                            (cc) (°F.)                                                                      (millidarcies)                                                                       Permeability                             __________________________________________________________________________    1  2% NaCl                                                                             Production                                                                          4.74  100  75  21.5   100                                      2  H.sub.2 O                                                                           Production                                                                          4.6    25  75  --     --                                       3  2% NaCl                                                                             Production                                                                          1.87   60  75  1.22    5.7                                     4  C     Injection                                                                           2.0    95  140 --     --                                       5  2% NaCl                                                                             Production                                                                          5.08  250  75  5.51   25.9                                     6  15% NaCl                                                                            Production                                                                          5.07  100  75  5.80   26.9                                     7  H.sub.2 O                                                                           Production                                                                          5.07  250  75  5.60   26.0                                     __________________________________________________________________________

                  TABLE III                                                       ______________________________________                                                               Flow                                                                          Rate  Vol-                                                          Flow      (cc/  ume  Temp. Permeability                          Step Fluid   Direction min)  (cc) (°F.)                                                                        (millidarcies)                        ______________________________________                                        1    Kero-   Production                                                                              3.0   400  75    5.65                                       sene                                                                     2    B       Injection 2.0   100  140   --                                    3    D       Injection 5.0   200  75    --                                    4    Kero-   Production                                                                              5.17  525  75    5.96                                       sene                                                                     5    15%     Production                                                                              2.62  200  75    2.22                                       NaCl                                                                     6    H.sub.2 O                                                                             Production                                                                              3.98  200  75    2.17                                  ______________________________________                                    

                  TABLE IV                                                        ______________________________________                                                               Flow                                                                          Rate  Vol-                                                          Flow      (cc/  ume  Temp. Permeability                          Step Fluid   Direction min)  (cc) (°F.)                                                                        (millidarcies)                        ______________________________________                                        1    2%      Production                                                                              2.2   400  75    0.35                                       NaCl                                                                     2    Kero-   Production                                                                              3.0   400  75    9.80                                       sene                                                                     3    B       Injection 2.0   100  140   --                                    4    2%      Production                                                                              2.2   100  75    3.30                                       NaCl                                                                     5    Kero-   Production                                                                              3.0   300  75    12.90                                      sene                                                                     ______________________________________                                    

EXAMPLE 7

A cylindrical core sample of a sandstone which contains fine particlesof kaolinite feldspar, quartz, and iron oxide is tested by a methodsimilar to that of Example 3. The sample is saturated with 3 percentsodium chloride solution, and an "overburden" pressure of about 1,000p.s.i.g. is used.

Results are summarized in Table V. Fluid C is as described in Example 4.Fluid "E" is an aqueous solution containing 10 percent hydrochloricacid, 2 percent hydrofluoric acid, 1.5 percent citric acid, 0.2 percentcorrosion inhibitor, and 1 percent 3-aminopropyltriethoxysilane. Fluid"F" is similar to Fluid C, except hydrochloric acid is only 5 percent.As shown in the table, the treatment significantly improves permeabilityof the core to both aqueous and organic materials.

EXAMPLE 8

A well is completed in an oil-bearing sandstone formation. Following aninitial daily production rate of 460 barrels oil and 800 barrels water,production declines at an annual rate about 86 percent, to 220 barrelsoil and 400 barrels water per day, at which point the well is shut in.The formation is determined to contain siderite fines, which may migrateand impair fluid permeability.

The well is treated to stabilize production rates. First, a sodiumbromide solution, containing a fluid loss agent, is used to kill thewell. Water is used to flush the solution out of the well. A 6,000gallon preflush slug of a high aromatics hydrocarbon solvent is theninjected into the formation. Next, 4,000 gallons of an aqueous solutioncontaining 10 percent hydrochloric acid, 10 percent acetic acid, 1percent 3-aminopropyltriethoxysilane, 0.3 percent corrosion

                  TABLE V                                                         ______________________________________                                                               Flow                                                                          Rate  Vol-                                                          Flow      (cc/  ume  Temp. Permeability                          Step Fluid   Direction min)  (cc) (°F.)                                                                        (millidarcies)                        ______________________________________                                        1    3%      Production                                                                              5.0   200   75   1.45                                       NaCl                                                                     2    Kero-   Production                                                                              5.0   200   75   1.42                                       sene                                                                     3    C       Injection 2.0   100  138   --                                    4    E       Injection 2.0   100  142   --                                    5    F       Injection 2.0   100  142   --                                    6    3%      Production                                                                              5.0   200   75   4.91                                       NaCl                                                                     7    Kero-   Production                                                                              5.0    50   75   25.2                                       sene                                                                     ______________________________________                                    

inhibitor, 10 percent ethylene glycol monobutyl ether, 0.5 percentsurfactant, and 5 percent citric acid (hereinafter called "acidsolution") are displaced into the formation with about 300 S.C.F. ofnitrogen. Following this, 1,000 gallons of 3 percent aqueous ammoniumchloride solution also containing 10 percent of a diverting agent(hereinafter called "salt solution") are displaced into the formationwith about 300 S.C.F. of nitrogen. Similarly, an additional 4,000gallons of acid solution, 1,000 gallons of salt solution, and 4,000gallons of acid solution are sequentially displaced with nitrogen intothe formation. Finally, 7,000 gallons of diesel fuel which contains 10%ethylene glycol monobutyl ether are displaced with nitrogen into theformation, as an afterflush.

The well is immediately produced by flowing and about 50 barrels offluid are recovered. A gas lift, with nitrogen, is used to recover theremaining injected fluids. After returning the well to pump production,a daily production rate of 200 barrels of oil and 250 barrels of wateris obtained. Six months later, the daily production rate remains at ahigh level, 175 barrels oil and 215 barrels water. The rate ofproduction decline has been reduced to about 24 percent per year, by thetreatment.

Various embodiments and modifications of this invention have beendescribed in the foregoing discussion and examples, and furthermodifications will be apparent to those skilled in the art. Suchmodifications are included within the scope of the invention as definedby the following claims.

What is claimed is:
 1. A method for acidizing a subterranean formation,comprising the steps of:(a) forming an acidizing composition by mixingan aqueous acid component with at least one water-soluble organosiliconcompound; and (b) injecting the composition into the formation.
 2. Themethod defined in claim 1 wherein the acid component is selected fromthe group consisting of mineral acids, organic acids, and mixturesthereof.
 3. The method defined in claim 2 wherein the acid component isa mineral acid, selected from the group consisting of hydrochloric acid,nitric acid, hydroiodic acid, hydrobromic acid, sulfuric acid, sulfamicacid, phosphoric acid, mixtures of any of the foregoing acids with oneor more water-soluble fluoride salts, fluoboric acid,hexafluorophosphoric acid, hydrofluoric acid, difluorophosphoric acid,fluorosulfonic acid, and mixtures thereof.
 4. The method defined inclaim 2 wherein the acid component is an organic acid, selected from thegroup consisting of formic acid, acetic acid, halogenated derivatives ofacetic acid, citric acid, propionic acid, tartaric acid, and mixturesthereof.
 5. The method defined in claim 1 wherein the organosiliconcompound is selected from the group consisting of water-solubleorganosilane compounds and organosilane compounds which hydrolyze inaqueous media to form water-soluble silanols.
 6. The method defined inclaim 5 wherein the organosilane compound is water-soluble.
 7. Themethod defined in claim 6 wherein the organosilicon compound is selectedfrom the group consisting of amino silanes and vinyl silane compounds.8. The method defined in claim 7 wherein the organosilicon compound is3-aminopropyltriethoxy silane.
 9. The method defined in claim 1 whereinthe injecting of step (b) is preceded by a preflushing step, in which asalt solution or a hydrocarbon liquid is injected into the formation.10. The method defined in claim 1 wherein the injecting of step (b) isfollowed by an afterflush step, in which an aqueous solution or ahydrocarbon liquid is injected into the formation.
 11. The methoddefined in claim 1 wherein the injecting of step (b) is preceded by apreflushing step, in which a salt solution or a hydrocarbon liquid isinjected into the formation, and is followed by an afterflush step, inwhich an aqueous solution or a hydrocarbon liquid is injected into theformation.
 12. A method for acidizing a subterranean formation,comprising the steps of:(a) forming an acidizing composition by mixingthe components:(i) an aqueous solution of an acid selected from thegroup consisting of mineral acids, organic acids, and mixtures thereof;and (ii) an organosilicon compound selected from the group consisting ofwater-soluble organosilane compounds and organosilane compounds whichhydrolyze in aqueous media to form water-soluble silanols; and (b)injecting the composition into the formation.
 13. The method defined inclaim 12 wherein the acid is a mineral acid, selected from the groupconsisting of hydrochloric acid, nitric acid, hydroiodic acid,hydrobromic acid, sulfuric acid, sulfamic acid, phosphoric acid,mixtures of any of the foregoing acids with one or more water-solublefluoride salts, fluoboric acid, hexafluorophosphoric acid, hydrofluoricacid, difluorophosphoric acid, fluorosulfonic acid, and mixturesthereof.
 14. The method defined in claim 12 wherein the acid is anorganic acid, selected from the group consisting of formic acid, aceticacid, halogenated derivatives of acetic acid, citric acid, propionicacid, tartaric acid, and mixtures thereof.
 15. The method defined inclaim 12 wherein the organosilane compound is selected from the groupconsisting of amino silanes and vinyl silane compounds.
 16. The methoddefined in claim 15 wherein the organosilane compound is3-aminopropyltriethoxy silane.
 17. The method defined in claim 12wherein the injecting of step (b) is preceded by a preflushing step, inwhich a salt solution or a hydrocarbon liquid is injected into theformation.
 18. The method defined in claim 12 wherein the injecting ofstep (b) is followed by an afterflush step, in which an aqueous solutionor a hydrocarbon liquid is injected into the formation.
 19. The methoddefined in claim 12 wherein the injecting of step (b) is preceded by apreflushing step, in which a salt solution or a hydrocarbon liquid isinjected into the formation, and is followed by an afterflush step, inwhich an aqueous solution or a hydrocarbon liquid is injected into theformation.
 20. A method for acidizing a subterranean formation,comprising the steps of:(a) preflushing the formation by injecting asalt solution or a hydrocarbon liquid; (b) forming an acidizingcomposition by mixing the components:(i) an aqueous solution of an acidselected from the group consisting of mineral acids, organic acids, andmixtures thereof; and (ii) a water-soluble organosilane compoundselected from the group consisting of 3-aminopropyltriethoxy silane,N-2-aminoethyl-3-aminopropyltrimethoxy silane, vinyltris-(2-methoxyethoxy)silane, and mixtures thereof; and (c) injectingthe composition into the formation.
 21. The method defined in claim 20wherein the acid is a mineral acid, selected from the group consistingof hydrochloric acid, nitric acid, hydroiodic acid, hydrobromic acid,sulfuric acid, sulfamic acid, phosphoric acid, mixtures of any of theforegoing acids with one or more water-soluble fluoride salts, fluoboricacid, hexafluorophosphoric acid, hydrofluoric acid, difluorophosphoricacid, fluorosulfonic acid, and mixtures thereof.
 22. The method definedin claim 20 wherein the acid is an organic acid, selected from the groupconsisting of formic acid, acetic acid, halogenated derivatives ofacetic acid, citric acid, propionic acid, tartaric acid, and mixturesthereof.
 23. The method defined in claim 20 wherein the injecting ofstep (c) is followed by an afterflush step, in which an aqueous solutionor a hydrocarbon liquid is injected into the formation.
 24. The methoddefined in claim 20 wherein the organosilane compound is3-aminopropyltriethoxy silane.
 25. A method for acidizing a subterraneanformation comprising injecting into the formation a compositioncomprising:(a) about 0.5 to about 50 percent by weight of an aqueousacid component selected from the group consisting of mineral acids,organic acids, and mixtures thereof; and (b) about 0.1 to about 10percent by weight of at least one water-soluble organosilicon compound.